Monitor the columnar engine

This page describes how to monitor utilization of the columnar engine.

Verify usage of the columnar engine using EXPLAIN

You can verify the usage of the columnar engine by using the EXPLAIN statement to observe the new columnar operators that appear in a query's generated query plan.

psql Client

EXPLAIN (ANALYZE,COSTS OFF,BUFFERS,TIMING OFF,SUMMARY OFF)
  SELECT l_returnflag, l_linestatus, l_quantity, l_extendedprice,
         l_discount, l_tax
    FROM lineitem
   WHERE l_shipdate <= date '1992-08-06'
;
                                 QUERY PLAN
-----------------------------------------------------------------------------
 Append (actual rows=3941797 loops=1)
   Buffers: shared hit=9
   ->  Custom Scan (columnar scan) on lineitem (actual rows=3941797 loops=1)
         Filter: (l_shipdate <= '1992-08-06'::date)
         Rows Removed by Columnar Filter: 56054083
         Columnar cache search mode: columnar filter only
         Buffers: shared hit=9
   ->  Seq Scan on lineitem (never executed)
  Filter: (l_shipdate <= '1992-08-06'::date)
  • Custom Scan (columnar scan) indicates that columnar-engine scanning is being included in the query plan.
  • Rows Removed by Columnar Filter lists the number of rows filtered out by the columnar vectorized execution.
  • Columnar cache search mode can be columnar filter only, native, or row store scan. The planner chooses the search mode automatically based on costing and pushdown evaluation capability.

When the planner chooses the native mode, it pushes down some of the columnar operators to the scan:

  • Rows Aggregated by Columnar Scan lists the number of rows that are aggregated.
  • Rows Sorted by Columnar Scan lists the number of rows that are sorted.
  • Rows Limited by Columnar Scan lists the limited number of rows that were scanned.

With joins, columnar scan operators can also use the Late Materialization mode.

psql Client

EXPLAIN (ANALYZE,COSTS OFF,BUFFERS,TIMING OFF,SUMMARY OFF)
  SELECT l_shipmode, o_orderpriority
    FROM orders, lineitem
   WHERE o_orderkey = l_orderkey
         AND l_shipmode in ('AIR', 'FOB')
         AND l_receiptdate >= date '1995-01-01'
;
                                 QUERY PLAN
-----------------------------------------------------------------------------
 Hash Join (actual rows=9865288 loops=1)
   Hash Cond: (lineitem.l_orderkey = orders.o_orderkey)
   Buffers: temp read=127738 written=127738
   ->  Append (actual rows=9865288 loops=1)
         ->  Custom Scan (columnar scan) on lineitem (actual rows=9865288 loops=1)
               Filter: ((l_shipmode = ANY ('{AIR,FOB}'::bpchar[])) AND
               (l_receiptdate >= '1995-01-01'::date))
               Rows Removed by Columnar Filter: 50130592
               Columnar cache search mode: native
         ->  Index Scan using idx_lineitem_orderkey_fkidx on lineitem
             (never executed)
               Filter: ((l_shipmode = ANY ('{AIR,FOB}'::bpchar[])) AND
               (l_receiptdate >= '1995-01-01'::date))
   ->  Hash (actual rows=15000000 loops=1)
         Buckets: 1048576  Batches: 32  Memory Usage: 37006kB
         Buffers: temp written=83357
         ->  Append (actual rows=15000000 loops=1)
               ->  Custom Scan (columnar scan) on orders (actual rows=15000000
                   loops=1)
                     Rows Removed by Columnar Filter: 0
                     Columnar projection mode: late materialization
                     Columnar cache search mode: native
               ->  Seq Scan on orders (never executed)

Columnar projection mode can be late materialization. Columnar operators choose this mode automatically when the planner optimizes the projection by deferring the materialization of some column values.

View information about tables with columns in the column store

You can view information about the tables with columns in the column store by querying the g_columnar_relations view.

psql Client

SELECT * FROM g_columnar_relations;

┌─[ RECORD 1 ]────────┬───────────────────┐
│ relation_name       │ tbl_parallel_test │
│ schema_name         │ public            │
│ database_name       │ advisor           │
│ status              │ Usable            │
│ size                │ 581431259         │
│ columnar_unit_count │ 3                 │
│ invalid_block_count │ 0                 │
│ total_block_count   │ 8337              │
├─[ RECORD 2 ]────────┼───────────────────┤
│ relation_name       │ lineitem          │
│ schema_name         │ public            │
│ database_name       │ advisor           │
│ status              │ Usable            │
│ size                │ 423224944         │
│ columnar_unit_count │ 29                │
│ invalid_block_count │ 0                 │
│ total_block_count   │ 115662            │
├─[ RECORD 3 ]────────┼───────────────────┤
...